It is well known to provide power assistance to the steering sub-systems of driver-operated vehicles.
In various types of vehicle it has for many years been commonplace, especially in the case of relatively large vehicles in which the required steering effort may be significant, to provide power assistance in the form of a hydraulic servomechanism. This includes a pump for pressurising a fluid such as hydraulic oil in a circuit that includes a servovalve. The degree of opening of the servovalve is determined by turning of the steering column, of the vehicle, that is coupled to an input shaft of the valve by way of a mechanical linkage.
The valve pilots the hydraulic fluid pressure, in dependence on the resulting valve setting, to an assist motor that in some applications is a linear hydraulic cylinder having at each end an extensible arm and in some other applications may be e.g. a hydrostatic motor.
The cylinder when fitted interconnects by way of the ends of the arms the steerable wheels of the vehicle in replacement of conventional, solid steering linkage members. The cylinder may act in the same manner as the rigid linkages or, when extension of the arms occurs as a result of operation of the servomechanism, to reduce the effort needed to turn the steering wheel against the resistance provided by the mass of the vehicle acting through the steerable wheels.
Power assistance of this general kind renders the feeling of the steering “light” (i.e. relatively effortless) as experienced by the driver of a vehicle equipped with it. Moreover if the servomechanism fails for any reason the cylinder becomes rigid and behaves in the same manner as a conventional linkage member. In consequence it remains possible to steer the vehicle, albeit through the application of considerably more effort on the part of the driver, in the event of a servomechanism fault developing.
One characteristic of hydraulic servomechanisms of the general kind described above however is that they do not on their own provide a “self-centring” characteristic. This is of course important in most self-propelled, driver-controlled vehicles since it is strongly desirable that the steerable wheels return to a “straight ahead” setting in the event of the driver releasing a steering input member (that in a passenger car and the majority of the vehicle types listed is a steering wheel).
Passenger vehicle steering is normally designed to incorporate a positive caster angle that gives rise to a mechanical self-centring effect. This tends to provide directional stability when the vehicle is driven on relatively smooth, metalled roads as is normally the case for such a vehicle.
A self-centring steering effect is of limited benefit in the case of agricultural vehicles when they are operating in off-road situations such as when driving in fields.
This is partly because furrows or other features of e.g. a ploughed field tend to override any self-centring effect brought about through design features including positive camber angles. The forces induced by the furrows on the steerable wheels (or other ground-engaging members of the tractor, if fitted) exceed any self-centring forces induced by the steering geometry.
Moreover under many circumstances when operating in a field a tractor operator may find a self-centring effect to be inconvenient. Examples arise when turning at a headland, or when following a curved margin of a field, when driving on lateral slopes (in a so-called “crab-like motion”), or when ploughing with two wheels in a furrow.
At such times it may be desirable in effect to “set” a steering angle for an extended period of operation. The tractor driver therefore may find it bothersome to have to adjust the steering input member continuously in order to overcome a self-centring effect and assure constancy of the steering angle.
In any event a modern tractor is a complicated multi-purpose vehicle that may be fitted with or coupled to numerous sub-systems (such as harvesters, spraying equipment and tilling implements) that require operation from within the operator's cab. When operating such sub-systems the driver may find it very difficult simultaneously to make steering input adjustments.
On the other hand when moving in a straight line on a relatively flat field, or when driving on roads, a tractor operator may find it desirable for the tractor to behave in ways that are similar to other road vehicles, and thereby exhibit a self-centring steering effect or other effects that are useful from the standpoints of safety, stability and/or convenience.
Another known type of power steering assistance mechanism is an electric type, in which rotations of the steering column are converted (e.g. using a Hall effect device, or another kind of sensor) into electrical signals. These in turn may be used to generate commands for an electric motor that is connected to provide rotational assistance forces to the steering column. Indeed in some arrangements the shaft of the steering column itself is constituted over part of its length as the rotor of the motor.
It would be desirable to be able to adjust the characteristics of the steering of a large, heavy vehicle that operates under varying circumstances, to take account of the prevailing type of use. The presence or absence of a self-centring steering effect is exemplary of the kind of characteristic it could be desirable to adjust, although there exist others such as but not limited to the degree of damping of the steering (i.e. the extent to which the steering deviates from a set steering angle); the extent to which power assistance applies to assist the vehicle operator; assistance in turning from one lock to the opposite lock as quickly as possible; and more general transitions of characteristics that take account of changes from rough off-road to on-road or smooth field driving.